1. Field of the Invention
The present invention relates to a magnetoresistance device and a magnetic memory using the same. Particularly, the present invention relates to a magnetoresistance device provided with a free magnetic layer incorporating a plurality of ferromagnetic layers, adjacent two of which are antiparallelly coupled.
2. Description of the Related Art
In recent yeas, research and develop activity in the field of the MRAM (Magnetic Random Access Memory) has been enhanced, due to the fast write and read access. FIG. 1A illustrates an exemplary structure of a typical MRAM. The MRAM shown in FIG. 1 is composed of a substrate 1, word lines 102 and bit lines 103 (each one shown), and magnetoresistive elements (one shown) each including an antiferromagnetic layer 3, a fixed magnetic layer 4, a non-magnetic layer 5, a free magnetic layer 5. The magnetoresistive elements are arranged at respective intersections of the word lines 102 and the bit lines. The fixed magnetic layer 4 has a fixed magnetization, and the free magnetic layer 6 has a reversible magnetization. The write operation is typically achieved by generating current-induced magnetic field through feeding write currents on the selected word and bit lines 102 and 103, and thereby reversing the magnetization of the free magnetic layer 6 within the selected magnetoresistive element. Alternatively, the write operation may be achieved by directly injecting a spin-polarized current into the free magnetic layer 6 within the selected magnetoresistive element. The read operation is achieved by detecting the resistance change in the magnetoresistive elements caused by the magnetoresistance effect, such as TMR effect (tunneling magnetoresistance effect, and GMR effect (giant magnetoresistance effect).
The following is a description of the operation principle of the MRAM that achieves the write operation through generating a current-induced magnetic field. A rectangular magnetic layer with a magnetization of M, a thickness of t, and a uniaxial crystalline anisotropy energy of K exhibits a shape anisotropic magnetic field of cM×t/W and a crystalline anisotropy field Hk of K/2M, where W is a length of the narrow sides of the rectangular magnetic layer, and c is a coefficient dependent on the shape and aspect ratio of the rectangular magnetic layer. In an assumption that the longitudinal direction of a magnetic layer is identical to the easy axis direction thereof, which is usually the case within a magnetoresistive element, the magnetic layer exhibits uniaxial magnetic anisotropy, and has a, net anisotropy field Hka expressed by the following formula (1):
                                          H            ka                    =                                    cM              ×              t              ⁢                              /                            ⁢              W                        +                          k              ⁢                              /                            ⁢              2              ⁢              M                                      ,                                  ⁢                                  ⁢                  =                                    cM              ×              t              ⁢                              /                            ⁢              W                        +                          H              k                                      ,                                  ⁢                                  ⁢                  =                      Ka            ⁢                          /                        ⁢            2            ⁢            M                          ,                            (        1        )            where Ka is the net anisotropy energy.
As known in the art, the magnetization reversal in a magnetic layer having a uniaxial magnetic anisotropy ideally follows the Stoner-Wohlfarth model. In the Stoner-Wohlfarth approach, the magnetization is reversed at a threshold magnetic field, when the external magnetic field along the in-plane direction is increased. FIG. 3 is a diagram indicating a magnetic field region in which the magnetization reversal occurs when a magnetic field is applied to a magnetic layer; the X axis is defined along the easy axis of the magnetic layer and the Y axis is defined along the hard axis. The threshold magnetic field is represented by an asteroid curve in FIG. 3. When the external magnetic field falls in the region outside the asteroid curve, the magnetization of the magnetic layer is flipped in the direction along the easy axis.
A MRAM typically achieves selective data write operation into the selected memory cell (or selected magnetoresistive element) by using this phenomenon. FIG. 2 illustrates an exemplary layout of an MRAM such designed. MTJ (magnetic tunnel junction) elements 101 (one shown) are formed at respective intersections of word lines 102 and bit lines 103. The MTJ elements 101 are each formed on a lower electrode pattern 107 connected with a lower via contact 108 which provides an electric connection to the MTJ element 101. In this MRAM, a synthetic magnetic field generated by the selected bit and word lines is applied to the selected MTJ element 101, and the magnetization of the free magnetic layer within the selected MTJ element 101 is selectively reversed.
Another data writing technique is to directly inject a spin-polarized current into the free magnetic layer 6. The spin-polarized current exerts a torque on the magnetization of the free magnetic layer to achieve magnetization reversal. When a current is fed to the magnetoresistive element shown in FIG. 1A in the cross-plane direction, a spin-polarized current is generated through the non-magnetic layer 5, and a spin torque is transferred between the free magnetic layer 6 and the fixed magnetic layer 4 to achieve the magnetization reversal of the free magnetic layer 6. The direction of the magnetization is controllable by selecting the direction of the current. This may be referred to as the spin-polarized current switching. The MRAM using the spin-polarized current switching is superior in terms of the reduced write current and write data error.
A stack of multiple ferromagnetic layers separated by one or more non-magnetic layers in which adjacent two of the ferromagnetic layers are antiferromagnetically coupled is often used in a magnetoresistance device, such as memory cells in MRAMs and magnetic heads in magnetic disc drives. Such stack is often referred to as a synthetic antiferromagnet or simply “SAF”. The SAF is useful for reducing and controlling the demagnetizing field generation accompanied by miniaturization in the magnetoresistive element. The SAF may be used for the fixed magnetic layer and/or the free magnetic layer within a magnetoresistive element. FIG. 1B illustrates a magnetoresistive element structure in which a SAF is used as the free magnetic layer 6. The free magnetic layer 6 is composed of two ferromagnetic layers 104 and 106 separated by a non-magnetic layer 105, and the magnetizations, denoted by numerals m1 and m2, of the ferromagnetic layers 104 and 106 are antiparallelly oriented, when no external field is applied to the free magnetic layer 6.
The magnetization reversal behavior of the free magnetic layer within the SAF is described in the following. For simplicity, the shape magnetic anisotropy is disregarded in the following explanation. The behavior of the SAF is different between the case when the SAF has an unignorable net magnetization and the case when the net magnetization of the SAF is ignorable.
Firstly, the behavior of the SAF is described for the case where the SAF has an unignorable net magnetization. Specifically, the following discussion is directed to the case when the SAF is composed of two ferromagnetic layers made of different materials and/or having different thicknesses, assuming that the two ferromagnetic layers have magnetizations of M1 and M2 and thicknesses of t1 and t2, respectively, and the following relation holds:M1t1>M2t2,wherein the two ferromagnetic layers are antiferromagnetically coupled with an exchange coupling energy JSAF. When a magnetic field applied in the direction of the easy axis is increased from the zero magnetic field, as shown in FIG. 4, the antiparallel coupling between the magnetizations M1 and M2 starts to be decoupled at a certain magnetic field H1. When the magnetic field is further increased, the magnetizations M1 and M2 are oriented in parallel. The minimum magnetic field that orients the magnetizations M1 and M2 in parallel is referred to as the saturation magnetic field HS. The above-mentioned magnetic field H1 and the magnetic field HS are respectively represented by the following formulas (2), (3):H1=JSAF[1/(((M2t2)−1/(M1t1)],  (2)HS=JSAF[1/((M2t2)+1/(M1t1)],  (3)
The magnetic fields H1 and HS depend on the exchange coupling energy, the saturation magnetizations and thicknesses of the two ferromagnetic layers within the SAF.
Next, a description is given of the case when the SAF only has an ignorable net magnetization. Specifically, the following discussion is directed to the case when the SAF is composed of two identically structured ferromagnetic layers coupled with the exchange coupling energy JSAF. When the ferromagnetic layers have the same magnetization of M, and the same thickness of t (that is, when M1=M2=M and t1=t2=t) and have the same crystalline anisotropy energy of K, and the crystalline anisotropy magnetic field Hk of the ferromagnetic layers is expressed by K/2M. In this case, the SAF exhibits a magnetization curve shown in FIG. 5, when an external magnetic field is applied in the easy axis direction. Although exhibiting the zero net magnetization with the zero external magnetic field applied to the SAF, the SAF suddenly exhibits a non-zero net magnetization, when the external magnetic field applied in the easy axis direction is increased up to a certain magnetic field Hflop. At this time, the magnetizations of two ferromagnetic layers are magnetically coupled each other, directed at a certain angle so that the net magnetization of the SAF is oriented in the direction of the external magnetic field. Such phenomenon is often referred to as the spin flop, and the magnetic field Hflop that causes the spin flop is referred to as the spin-flop field. Note that the spin flop occurs only when the net magnetization of the SAF with no external field applied is sufficiently small. The magnetizations of the two ferromagnetic layers are finally oriented in parallel, when the magnetic field if further increased. The magnetic field that orients the magnetizations of the two ferromagnetic layers in parallel is referred to as the saturation magnetic field HS. The spin-flop field Hflop and the saturation magnetic field HS are respectively expressed by the following formulas (4), (5):
                                                                                          H                  flop                                =                                ⁢                                  2                  ⁢                                      /                                    ⁢                                                            M                      ⁡                                              [                                                  K                          ⁡                                                      (                                                                                                                            J                                  SAF                                                                ⁢                                                                  /                                                                ⁢                                t                                                            -                              K                                                        )                                                                          ]                                                              0.5                                                                                                                                            =                                    ⁢                                                            (                                                                        H                          s                                                -                                                  H                          k                                                                    )                                        0.5                                                  ,                                                    ⁢                                  ⁢        and                            (        4        )                                                                                    H                S                            =                            ⁢                                                2                  ⁢                                      J                    SAF                                    ⁢                                      /                                    ⁢                                      (                    Mt                    )                                                  -                                  2                  ⁢                                      K                    /                    M                                                                                                                          =                            ⁢                                                2                  ⁢                                      J                    SAF                                    ⁢                                      /                                    ⁢                                      (                    Mt                    )                                                  -                                                      H                    k                                    .                                                                                        (        5        )            
As is apparent from the formula (4), the spin-flop field Hflop is uniquely determined by the saturation magnetic field HS and the anisotropy field Hk. The spin-flop field Hflop is the magnetic field where the antiparallel coupling between the two ferromagnetic layers within the SAF film starts to be decoupled, as is the case of the above-described magnetic field H1.
An MRAM that uses the spin flop of the SAF for data write operations is disclosed in U.S. Pat. No. 6,545,906. FIG. 6 illustrates the structure of the disclosed MRAM. The longitudinal direction of a MTJ 201 is inclined at an angle of 45 degrees to a word line 202 and a bit line 203. FIG. 8 illustrates the write operation procedure of the MRAM shown in FIG. 6. It should be noted that the magnetizations of respective ferromagnetic layers within the free magnetic layer of the MTJ 201 is denoted by symbols “M1”, and “M2” in FIG. 8.
The data write operation of the MRAM involves orienting the magnetizations of the ferromagnetic layers within the free magnetic layer to desired directions by rotating the in-plane direction of a magnetic field applied to the free magnetic layer of the MTJ 201. Specifically, a write current is firstly fed to the word line 202 to thereby generate a magnetic field HWL in the direction perpendicular to the word line 202 at Time t1. This is followed by feeding another write current to the bit line 203 at Time t2, with the write current continuingly fed to the word line 202. As a result, a magnetic field HWL+HBL is generated in the oblique direction with respect to the word line 202 and the bit line 203, typically, at an angle of 45 degrees to the word line 202 and the bit line 203. This is further followed by terminating feeding the writing current to the word line 202 at Time t3, while the writing current is continued to be fed to the bit line 203. This results in that the magnetic field HBL is generated in the direction perpendicular to the bit line 203 (that is, the direction parallel to the word line 202). The operation thus described achieves rotation of the magnetic field applied to the free magnetic layer through feeding the write currents to the word line 202 and the bit line 203 to thereby rotate the magnetizations of the ferromagnetic layers within the free magnetic layer of the SAF by 180 degrees. Hereinafter, the thus-described data write operation may be referred to as the “toggle writing”.
The MRAM based on the “toggle writing” requires that the magnetic field applied to the free magnetic layer generated by the writing currents through the word line 202 and the bit line 203 be larger than above-defined spin-flop field Hflop, and be smaller than the saturation magnetic field HS; otherwise, the magnetizations of the ferromagnetic layers within the free magnetic layer are not reversed to desired directions.
The MRAM write operation based on the toggle writing has various advantages. One advantage is high memory cell selectivity. FIG. 7 illustrates an exemplary operation region in which the magnetizations within the SAF are reversed by the magnetic fields generated by the write currents fed to the word line 202 and the bit line 203. In principle, the magnetizations within the SAF are not reversed in the toggle writing, when only one of the word line 202 and the bit line 203 is fed with a write current. In other words, the magnetizations within a non-selecting memory cell are not undesirably reversed. This is important in view of the reliability of the operation of the MRAM.
As shown in FIG. 7, the toggle writing-based MRAM exhibits an operation region along the toggling threshold curve on the side of the lower magnetic fields, in which the magnetizations within the SAF are directly switched to “1” or “0” state regardless of the original state of the SAF. This operation region is referred to as the direct mode reversal region. The size of the direct mode reversal region is increased as the difference |M1·t1−M2·t2| between magnetization-thickness products of the two ferromagnetic layers within the SAF is increased, and the state of the SAF after the direct mode reversal is determined depending on the dimension of the magnetization-thickness products of the two ferromagnetic films within the SAF, where the magnetization-thickness product of a ferromagnetic layer is defined as being the product of the saturation magnetization of the ferromagnetic layer and the film thickness thereof. This direct mode reversal region may be also used for write operations.
Reduction in the magnetic field for data writing in the toggle-writing-based MRAM may be achieved by reducing the spin-flop field Hflop (which is expressed by the formula (4)), for example, through reducing the exchange coupling energy JSAF. This is however, accompanied by a problem that the writing margin is also reduced. In the toggle writing, a successful write operation is achieved, when the synthetic magnetic field generated by the bit line and the word line ranges in a switching region between the spin-flop field Hflop and the saturation magnetic field HS. Therefore, a ratio defined by HS/Hflop can be considered to represent the writing margin. The writing margin HS/Hflop is expressed by the formula (6):HS/Hflop=Hflop/Hk.  (6)
As is apparent from the formula (6), the writing margin is reduced as the decrease in the spin-flop field Hflop, since the minimum crystalline anisotropy field Hk is limited to a certain range. Therefore, the reduction in the spin-flop field Hflop undesirably causes erroneous write operation of the toggle-writing-based MRAM. Therefore, an effort for reducing the write currents inevitably faces limit in a conventional toggle-writing-based MRAM. It would be advantageous if a toggle-writing-based MRAM is provided in which the spin-flop field Hflop is reduced with the ratio HS/Hflop increased.
In other MRAMs adopting other data write techniques (such as an MRAM that achieves data writing through switching the magnetization of a free magnetic layer by a synthetic magnetic field generated by word and bit lines, and an MRAM that achieves data writing through spin-polarized current injection), it would be also advantages if an SAF has a sufficiently large saturation field with a reduced net magnetization, thereby reducing the switching field of the SAF. The enhancement of the antiferromagnetic coupling in the SAF with the net magnetization reduced effectively reduces the effective shape magnetic anisotropy of the free magnetic layer, and helps single domain formation in the free magnetic layer. This effectively reduces the magnetization switching field of the SAF, and also reduces variations in the magnetization switching field.
U.S. Pat. No. 6,714,446 discloses an SAF structure that minimizes the corresponding increase in current needed to alter the magnetization direction. The disclosed SAF structure incorporates two multi-layer structures and a space layer interposed therebetween, each of the two multi-layer structures being composed of two magnetic sublayers and a spacer layer interposed therebetween. However, the disclosed SAF structure does not address maintaining or improving the saturation field of the SAF.